Garcelon was born in Lewiston (in modern-day Maine, then a part of Massachusetts), to French Huguenot parents.

During the impeachment of Andrew Johnson after the War, Garcelon became disgusted with the Republican Party and their policy of "Radical Reconstruction" and became a Democrat.

During his term as governor, Garcelon oversaw the "Greenback" controversy, when he investigated alleged voter fraud and determined that the Democrats and not the Republicans had won a majority in the legislature.

Senator James Blaine came to Augusta with a hundred armed men to protest the results, and Garcelon called out the state militia.

Civil war was narrowly averted, thanks to the peaceful intervention of militia leader Joshua Lawrence Chamberlain.