As a lieutenant in Starfleet, Margaret Alexander (known familiarly as "Madge") served for one year aboard the USS Enterprise under Captain James T. Kirk in the 2260s. Her performance aboard the Enterprise was such that she earned a field promotion, and transferred to the USS Firenze to serve as its second officer.
Later, she was promoted to the rank of Commander, and was assigned as executive officer of the USS Freedom. She then was promoted to Captain and took command of the Freedom, a position she held until that ship was decommissioned.
By 2285, she commanded the Miranda-class USS Saratoga. In 2286, Sinclair's Saratoga was assigned for a key mission to the Romulan Neutral Zone, carrying Captain Styles who was sent by Starfleet to relieve James T. Kirk from command of the USS Excelsior.
Later that year, she took Saratoga on a patrol of the area near the Zone, where they encountered, and were disabled by, the Whalesong probe.
She was still commanding the Saratoga in 2289 when it was to bring Sarek to a Klingon-Federation peace conference, but was substituted for by the USS Excelsior.
By 2293, Sinclair-Alexander had risen to the rank of Admiral, assigned to Starfleet Headquarters. That year, she persuaded Kirk to take part in the launch ceremonies for the new USS Enterprise-B.
After Kirk was apparently killed aboard that vessel, she spoke his memorial service in the grounds of Starfleet Academy, where she recalled the time she spent serving with Kirk and all she had learned from him.
By 2311, she had risen to the position of Commander-in-Chief of Starfleet. That year, she approved Captain John Harriman's plan to capture the Romulan flagship Tomed, and also recruited both Drysi Gravenor and Los Tirasol Mentir to assist with the mission.
Margaret Alexander met Cynthia Sinclair sometime after leaving command of the Saratoga. The two were married in 2292, and Alexander changed her surname to Sinclair-Alexander.
The captain of the Saratoga was unnamed in the film. The surname "Alexander" was established in the novelization. Serpents author David R. George III added her first and married names, in tribute to the actress who played the role, Madge Sinclair.