Mary Mc Donnell is the character Laura Roslin is also the president of the Twelve Colonies, until her death on the series finale.

Roslin has started off as the Secretary of Education in BSG, who has been working with the President Richard Adar. She was working with the President, ever since he was a Mayor in Caprica City. The episodes show that Roslin had some romantic relationship with the President. It disappeared after the diffusion of a teachers’ strike, large scale.

As it happens, Adar didn’t trust Roslin’s ability, when she was asked to resign. Adar, the president, is thinking to use force, to eventually stop the striking teachers, so it can also discourage the unions in Caprica.
Roslin was adamant, and wouldn’t want to resign. She visited her doctor’s appointment before attending a final ceremony. Roslin was shown to have a breast cancer, and a year to live. Upon leaving, the Cylons finally attacked. When she had to answer an automated signal, she was informed that she is now the highest surviving official in line of the Presidential line of succession, and is immediately sworn in as President.