In his later teen years (early 1960s) he made the rounds of the local clubs and bars in his area and worked with a number of bands (even performing as a single artist on occasion).

During that period, he met vocalist Eddie Brigati and keyboardist Felix Cavaliere (also members of the Starliters).

By late 1964, Brigati, Cavaliere, and Cornish had teamed with drummer Dino Danelli to form a group that became known as The Young Rascals.

On April 24, 2010, all four members of The Rascals reunited for the Kristen Ann Carr benefit, which was held at New York's Tribeca Grill.

He reunited with his band-mates when The Rascals appeared at the Capital Theater in Port Chester, New York, for six shows in December 2012 and for fifteen dates at the Richard Rogers Theatre on Broadway (April 15 – May 5, 2013).

Their production, entitled 'Once Upon A Dream', toured North America (Toronto, Los Angeles, Phoenix, Chicago, Detroit, Rochester, and New York City).

On September 7, 2018, Cornish collapsed onstage at the Alberta Bair Theater in Billings, Montana, as the band began performing their first song of the evening.