He attended Saint Jerome"s School, All Hallows High School, and Georgetown University. He graduated from New York University School of Law in 1934.
He was admitted to the bar in 1935, and practiced law in New York City. McDonnell was elected on February 17, 1948, to the New York State Assembly (Bronx Company, 1st Doctorate), to fill the vacancy caused by the appointment of Patrick J. Fogarty to the Domestic Relations Court. McDonnell was re-elected several times, and remained in the Assembly until his death in 1959, sitting in the 166th, 167th, 168th, 169th, 170th, 171st and 172nd New York State Legislatures.
He died on August 1, 1959, at his home at 262 Alexander Avenue in the Bronx.