An Irish Catholic from Boston, Leo is a former alcholic and pill-popper (valium). He was Treasury Secretary presumably two administrations before the Bartlet administration, and is the one that convinced Bartlet to run in the first place. Leo was good friends with the Lymans, and is the one that brought Josh onto the campaign in the first place. He also made the decision to keep Toby on during the campaign, and sent Toby to find CJ in California.

He has a daughter named Mallory, who despite dating a hockey player, has an itch for Sam Seaborn. She is a public elementary school teacher and is violently against school vouchers. Leo was married to Jenny, but got divorced as a result of his absence at home due to work. He has two sisters, Elizabeth and Josphine (Josie). Josie works in education, and Leo has accused her of "strutting", which apparently, he does not do.

Leo flew planes in Vietnam and adds the military perspective to the administration, because he is the only staffer who has served in the Armed Forces. He is therefore in every meeting in the Situation Room, and his perspective is valued extremely highly in this area by the President. This causes some pundits to fear the power that Leo has, because while not elected, he sometimes is really the guy running the country.

He does not know who Charlie Brown is, thinks the missile defense shield is a good idea (and we still love him), and has this little annoyance problem with Lord John Marbury, the English ambassador. He bikes, has known the President for 40 years, is quite fond of Andrew Jackson, and saying "yeah". His official title is Chief of Staff, has been accused of bringing Leo too close to the middle politically, and is Bartlet's most trusted advisor. Margaret is his assistant who worries about him frequently. He has a temper, though most often is the benevolent father-figure on the staff.