Note that between 1990 and 2000 Croatia had a semi-presidential system and members of government, including Prime Ministers, were all directly appointed and removed by President of Croatia, the post held by Franjo Tuđman from 1990 to late 1999. During this period, Tuđman was also president of the ruling Croatian Democratic Union (HDZ) conservative party, with all Prime Ministers in this period coming from his party's ranks.
Following Tuđman's death in December 1999 and the subsequent January 2000 general election, the new centre-left parliamentary majority led by Ivica Račan amended the Constitution of Croatia in November 2000 and introduced a parliamentary system, greatly reducing President's executive powers. Therefore since Račan all Prime Ministers were leaders of parties (or coalition of parties) which held parliament majority according to results in the general election. Meanwhile, the post of president was made a non-party posting, which meant that Presidents who served after Tuđman (Stjepan Mesić from 2000 to 2010 and Ivo Josipović from 2010 to present) "froze" their party memberships upon getting elected for the entire duration of their terms.
The longest serving Prime Minister to date was Ivo Sanader who held the post for 2,022 days between December 2003 and July 2009, followed by Zlatko Mateša (1,542 days from November 1995 to January 2000) and Ivica Račan (1,426 days from January 2000 to December 2003).