New London — The administrative assistant who has stood sentry at City Hall for three city managers, and most recently for the past 18 months for the City Council, is leaving public employment after a 35-year career.

Gail McKissick, who started working for the city in 1978 when C. Francis Driscoll was city manger, will retire at the end of the month. She also worked in the personnel office and the City Clerk’s Office.

McKissick, who declined to talk about her decision to retire, worked directly for former town managers Richard Brown, Martin Berliner and Denise Rose. When the city changed its form of government, eliminating the city manager’s position, she became administrative assistant for the City Council. She was secretary to the council for 25 years.

“She is just the most amazing resource available to me. She has all the institutional history of city government,’’ said Council President Michael Passero. “Having stepped into the role as the first council president, having a person like Gail McKissick by your side was the best thing to happen.”

Passero said the City Council has begun the process of hiring a new assistant. Clark van der Lyke, who was city clerk from 1983 to 2001 and does consulting work for the city, will fill in until a replacement is made.