Born in London, England, he moved to the United States with his family as a child and settled in Charlestown, Massachusetts. Pasco attended Harvard University and then moved to Florida in 1859. He served as principal of the Waukeenah Academy, a school in Monticello, Florida, from 1860 to 1861.

When the American Civil War began, though he had only lived in the South for two years, Pasco joined the army of the Confederate States of America. He fought as a member of the Florida Third Volunteers. He was captured in Mississippi and imprisoned by the United States for the rest of the war. He was released in March 1865 and immediately returned to Florida to resume his post as principal of the Waukeenah academy. He resigned from that position in 1866 but remained in Florida, serving as clerk of Jefferson County from 1866 to 1868. He eventually became a prominent lawyer in the area.

In 1885, he was the President of a convention which made a new constitution for Florida. He was a member of the Florida State Assembly from 1886 to 1887 and briefly served as speaker in 1887.

In 1887, Pasco was elected to the U.S. Senate from Florida, as a member of the United States Democratic Party. He served in the Senate for two terms, until 1899, when he was defeated for reelection. He then became a member of the Isthmian Canal Commission, which decided that a canal should be built through the isthmus of Panama. He remained on this commission until 1905, when work on the canal began.

Pasco then retired from public life and moved back to Monticello. He died in Tampa, Florida and was buried in the Roseland cemetery in Monticello. Pasco County, Florida is named for him. Though the county was established and named in 1887 while Pasco was still alive, he is believed to have never visited it.