Born William Wendell Gilman III on May 24, 1988, in Westerly, Rhode Island, he was raised in Hope Valley, Rhode Island. Gilman began singing before he was in school, and gave his first public performance at age 7. At the age of 9, Gilman was discovered by Ray Benson of Asleep at the Wheel, who helped him record demos. Gilman was then signed to Epic Records Nashville in 2000.

On January 23, 2001, he was voted the European's most awarded music award in new country artist, becoming the youngest contender to ever win that prize. He was also given entry into the Guinness Book of World Records for being the youngest singer to reach #1 on the Billboard Top Country Albums chart. In February 2001, he was nominated for a Grammy Award for Best Male Country Vocal Performance for his debut single "One Voice" and was nominated for Best Country Song for the songwriters, David Cook and David Malloy. That same year, he appeared in the tribute album Country Goes Raffi, performing Raffi's famous song "Baby Beluga."

Gilman was featured on Welsh soprano Charlotte Church's album Dream A Dream and released a Christmas album in 2000 titled Classic Christmas, which was certified Gold by the RIAA. In 2003, Billy released his fourth album, Dare to Dream, which was also certified gold. Although its singles "Elisabeth" and "She's My Girl" both fell short of Top 40 on the Billboard country singles charts.

In 2004, Gilman took part of a reading for the then Broadway-bound musical A Tale of Two Cities (musical). He played "The Young Man," who is Madame DeFarge's brother.

His final album for Epic was entitled Music Through Heartsongs: Songs Based on the Poems of Mattie J.T. Stepanek. These songs were based on poems written by Mattie Stepanek, a poet with muscular dystrophy who died in 2004.

Gilman signed to Image Entertainment in 2005 for the release of his fifth album, Everything and More. A self-titled album followed in 2006.

Gilman lives in Hope Valley, Rhode Island with his parents Bill and Fran Gilman and his younger brother Colin, frequently flying back to Nashville to write and record.[citation needed]. He is also Celebrity Ambassador for the Muscular Dystrophy Association.