From 1979 to 1983 he specialized internal medicine at the Sisters of Charity Hospital and at the University Medical Center Hamburg-Eppendorf in Germany.

Reiner remained in this position until 1995 when he was appointed head officer of the "Department of Internal Medicine" at the University Hospital Centre, Zagreb.

During his tenure the University Hospital Centre Zagreb was expanded and received modern equipment, and is now one and a half times the size it was before Reiner stepped in.

He was the secretary of the Croatian Medical Association and is a member of the committee for the selection of new Fellows of the Royal Society.

Reiner was the first Croat who was the main author of the European guidelines for the treatment of dyslipidaemia published in 2011.

[1] From 1995 to 1998, Reiner was member of the executive board of the World Health Organization and was the second Croat to serve as such.

From 2011 to 2012 Reiner was vice president of the Committee for Science, Education and Culture at the Croatian Parliament.

During the same session of Parliament, however, Reiner was once again elected as one of the Deputy Speakers from the ranks of the governing parliamentary majority.

In his capacity as a Deputy Speaker of Parliament, Reiner temporarily assumed the powers and duties of the President of Croatia on 11 July 2018 when, due to the rare simultaneous absence of both President Kolinda Grabar-Kitarović, who was attending the 2018 NATO summit in Brussels, and Speaker of Parliament Gordan Jandroković, who (along with Prime Minister Andrej Plenković) was attending the semi-finals of the 2018 World Cup in Moscow, he was left as the highest-ranking official eligible to take up the responsibility.

As the other two eligible Deputy Speakers, Milijan Brkić (HDZ) and Furio Radin (Italian national minority), were both also absent from the country, Reiner was left as the only possible choice to carry out the functions of head of state.

[2][3] Upon Jandroković's return to Croatia the following day, Reiner ceased to carry out the powers and duties of the presidency, which reverted to the Speaker.