Rachel Hubbard, county councillor for Lincoln Bracebridge, has resigned due to personal reasons.
Conservative councillor Hubbard was elected in June 2009.
In her four years with the council, she has also served as Executive Support Councillor to Cllr Peter Robinson, portfolio holder for Community Safety, Cohesion and Diversity, and as a member of the Economic Scrutiny Committee.
Cllr Martin Hill, Leader of Lincolnshire County Council, said: “It’s a real shame that she’s had to step down because of her health.
“A new councillor for Lincoln Bracebridge will be elected in May, but, in the meantime, Cllr Kelly Smith, who represents a neighbouring division, has agreed to fill in.”