When Attorney General Edward Brooke was elected to the United States Senate, Cowin joined him as a legislative assistant.
After Secretary of Administration and Finance Robert Yasi was appointed a judge in the Suffolk Probate Court, Cowin was chosen to succeed him.
[12] After the primary, Governor Sargent asked for state Republican chairman William Barnstead's resignation and recommended that Cowin succeed him.
[13] Sargent later postponed his efforts to remove Barnstead to focus on his race against Michael Dukakis.
[15] On September 19, 1978 he defeated State Representative Peter McDowell in the Republican Primary 128,914 votes to 86,250.