Ann-Ellen Hornidge, a partner at the law firm Mintz Levin with extensive experience in health care financing, will serve as chair of the Lahey Health System board of trustees, the Lahey Health System said Tuesday.

The appointment follows the alliance of Lahey Clinic of Burlington and Northeast Health System of Beverly, which formed a hospital network that stretches from Boston’s northwestern suburbs to Cape Ann. The network includes such hospitals as Lahey Clinic, Beverly Hospital, Addison Gilbert Hospital in Gloucester, and BayRidge Hospital in Lynn.

The plan to affiliate was disclosed in July 2011, and the affiliation was completed May 1.

Lahey Health System’s 13-member board is made up of four representatives selected by Northeast, four by Lahey, and four community representatives nominated by other trustees. Dr. Howard Grant, president and chief executive of both Lahey Health System and Lahey Clinic, also sits on the board.

Linda Moulton, chief executive of Ceralta Technologies Inc., will serve as vice chair of the Lahey Health System board.