Lalime was born in St. Bonaventure, Quebec. He played his junior hockey with the Shawinigan Cataractes of the QMJHL. In 1992-93, Lalime posted a 10-24-4 record with a GAA of 4.67 and a .863 save percentage as the team failed to make the playoffs. In the 1993 NHL Entry Draft, Lalime was chosen by the Pittsburgh Penguins in the 6th round, 156th overall pick. In 1993-94, he returned to the Cataractes and posted a solid 22-20-2 record with a GAA of 4.22. In the playoffs, Lalime struggled badly with a 1-3 record and GAA of 6.73.

In 1994-95, Lalime moved to the Hampton Roads Admirals of the ECHL where he posted a very solid 15-7-3 record, a GAA of 3.35 and save percentage of .898 until January of 1995. Then on January 26th, Lalime signed a contract with the Penguins, who then assigned him to the Cleveland Lumberjacks of the IHL. He struggled with the Lumberjacks, with a 7-10-4 record and a 4.44 GAA and save percentage of .882.

In 1995-96, Patrick remained with the Lumberjacks, and helped lead the team to the playoffs with a 20-12-7 record, 3.86 GAA and save percentage of .893 in 41 games. He was recalled to the Penguins twice in December 1995 and served as a backup goalie for 9 games, but never saw any actions. He also served as a backup for the Penguins in the playoffs for 4 games, and did not participate with Cleveland's playoff run.

Lalime went back to Cleveland for a 3rd season in 1996-97 and remained with the Lumberjacks until early November. The Penguins then called him up and Lalime made his NHL debut on November 16th against the New York Rangers, as he replaced Ken Wregget midway through the game, allowing 3 goals on 14 shots in an 8-3 loss. Lalime's next appearance was on December 6th, replacing Wregget after the 1st period down 2-0, but Lalime made 16 saves and the Penguins rebounded for a 5-3 win, giving him his first career victory. Lalime was given the start the next night against the Mighty Ducks of Anaheim, and he responded with 30 saves in a 5-3 victory. Lalime would go on to set an NHL record with the longest unbeaten streak to begin a career (16 games with a 14-0-2 record) before suffering his first defeat on January 23rd, a 4-3 OT loss to the Colorado Avalanche. During the streak, Lalime recorded his first shutout, a 4-0 win over the San Jose Sharks on December 13th, and had a 49 save performance against the Calgary Flames on January 21st, a 4-2 Penguins victory. After the streak ended, Lalime cooled off for the rest of the season and finished off with a 21-12-2 record, 2.94 GAA and a save percentage of .913. With Cleveland, Lalime was 6-6-2 with a 3.24 GAA. He dressed as the backup to Ken Wregget during the Penguins 5 playoff games that year.

In 1997-98, Lalime and the Penguins could not agree on a contract, and he spent the season with the Grand Rapids Griffins of the IHL, appearing in 31 games and posting a 10-10-9 record with a team leading GAA of 2.61 and .918 save percentage. On March 24, 1998, the Penguins traded his rights to the Anaheim Mighty Ducks for Sean Pronger, but Lalime stayed with the Griffins, and played in 1 playoff game, allowing 4 goals in an OT loss.

In 1998-99, he failed to make the Mighty Ducks team during the pre-season, and the club assigned him to the Kansas City Blades of the IHL. Lalime rewrote the teams record book with a 39-20-4 record, a 3.01 GAA and a save percentage of .900. He was named IHL goaltender of the month for March and earned a first team all-star selection after leading the league in wins, minutes played (3789) and saves (1708). In the playoffs, Lalime went 1-2 with a 2.08 GAA as Kansas City lost to the Long Beach Ice Dogs in the first round.

The Mighty Ducks traded Lalime to the Ottawa Senators for Ted Donato and Antti-Jussi Niemi on June 18th, and he opened the 1999-2000 season as a Senator as he and Ron Tugnutt would split duties. Lalime began his Sens career with a 3-0 shutout victory over the Philadelphia Flyers on October 2nd. He would go on and have a solid season with a 19-14-3 record, a GAA of 2.33 and save percentage of .905. The Sens would trade Tugnutt to Pittsburgh in March for Tom Barrasso, and Lalime would serve as his backup for the remainder of the season. Lalime dressed for 6 playoff games against the Toronto Maple Leafs, but did not see any action as the Sens were put out in the 1st round.

The Senators gave Lalime the starting job in 2000-01, and he responded with a 36-19-5 record, 2.35 GAA, and a save percentage of .914 and helped the Sens to winning the North-East Division, despite missing 10 games with a sprained left MCL in mid-October. He was named NHL Player of the Week twice (November 27-December 3 and February 12-18). In the playoffs, the Sens would meet the Leafs for the 2nd straight season, and Lalime played very solid, with a GAA of 2.39, however the Sens struggled to score goals and were eliminated in 4 games.

Lalime's 2001-02 season was a bit of a step back, but nonetheless was very respectable with a 27-24-8 record, 2.48 GAA and .903 save percentage. He was 2nd in the league with 7 shutouts and set a club record with a 149:41 shutout streak from October 23rd to November 10th. In the playoffs, the Sens faced off with the Philadelphia Flyers and were heavy underdogs, however, Lalime was unbeatable, as he allowed only 2 goals as Ottawa defeated the Flyers in 5 games. His GAA for the series was 0.40, and he had a .985 save percentage. In the 2nd round, the Sens met up with the Leafs for the 3rd straight year, and with a 5-0 game 1 win, Lalime tied an NHL playoff record with 4 shutouts. However the Leafs stormed back and eventually won the series in 7 games. Lalime finished the 2002 playoffs with a 7-5 record and a 1.39 GAA.

In 2002-03, the Senators won the Presidents Trophy for having the best regular season record, and Lalime led the way with a 39-20-7 record, 2.16 GAA and .911 save percentage. He was 2nd in the NHL in wins, shutouts (8), 4th in minutes (3943) and 5th in GAA. These stats, while remarkable, did not yield a Vezina Trophy nomination for the best NHL goaltender. Appeared in his first all-star game on February 2nd, making 18 saves on 19 shots in 25 minutes, then he allowed 3 goals during the shootout and took the loss. He broke his own record for shutout streak with 184:06 from January 9-18. In the playoffs, he helped the Sens defeat the New York Islanders in 5 games, then the Philadelphia Flyers in 6 games, before losing to the eventual Stanley Cup Champion New Jersey Devils in 7 games in the Eastern Conference Final. Lalime had an 11-7 record and GAA of 1.82 during the playoffs.

In 2003-04, Lalime struggled at times, as he finished with a 25-23-7 record, well short of expectations, with a 2.29 GAA and save percentage of .905. In the opening round of the playoffs, the Sens would meet their Battle of Ontario opponents, the Toronto Maple Leafs for the 4th time in 5 years. Lalime struggled during the series, allowing the opening goal in every game, and despite the Senators heavily outshooting the Leafs, the series was tied after 6 games. In the 6th game, Lalime played solid during overtime as the Senators defeated the Leafs to force game 7. However, in game 7, Lalime allowed 2 goals that a Junior C goalie could of stopped to Joe Nieuwendyk, burying the Senators, and after the 1st period he was pulled out of the game with the Sens losing 3-0. They would eventually lose 4-1 and lost to the Leafs for the 4th straight time in the playoffs.

The Senators decided to trade Lalime to the St. Louis Blues for a 4th round conditional pick in the 2005 NHL Draft on June 27th, 2004. Lalime was set to be the Blues starter during the 2004-05 season, however the lockout wiped out the season, and Lalime did not play any hockey that year.

During the 2005-06 season, he earned his first win with the Blues on October 11th against the Chicago Blackhawks, making 32 saves in a 4-1 victory. He was ultimately waived by the St. Louis on December 12th, 2005, after failing to make many saves expected of him in a 4-5 loss against the New York Rangers on December 10th, 2005. Lalime struggled with the Blues, recording a 3-11-4 record with a GAA of 3.97 and a save percentage of .866 in all the games leading up to the waive. After clearing waivers, Patrick was assigned to the Peoria Rivermen of the AHL. As of January 23rd, 2006, Lalime has a 5-4-1 record with Peoria, a GAA of 3.09 and save percentage of .881.

Lalime is known for his distinguishing goalie mask, featuring the Warner Brothers cartoon character Marvin the Martian. The use of the character may have been inspired by the similarity of Marvin's costume with the logo of Lalime's former team, the Ottawa Senators.