Since 1981, when Ronald Reagan was inaugurated, attorney general and the secretary of education have been two of the four longest serving Cabinet positions, on average. The other two are secretary of the interior and head of the Department of Health and Human Services. The position that sees the most turnover? Commerce.
Of the three twice-elected presidents who have completed their terms, Bill Clinton's Cabinet members stuck around the longest. (Obama and George H. W. Bush's cabinets have seen shorter tenures on average because neither reached eight years.)
The chart below shows the tenure of each of the members of the Cabinets of the past five presidents. (We included a few instances in which acting secretaries served for extended periods of time; they are shown as gaps. All year lengths were rounded to the nearest half-year.) There have been six Cabinet members who served a full eight years: Sam Pierce, Reagan's HUD secretary; Chao, under Bush; and Bill Clinton's secretaries of Education, Health and Human Services, and the Interior. Oh, and Clinton's attorney general, Janet Reno, who outlasted Eric Holder under perhaps equally trying circumstances.