International Business Machines Corp said on Monday Steven Mills, executive vice president, software and systems, decided to retire on Dec. 31, after a career spanning more than four decades.

Mills, 64, played a leading role in the growth of IBM Software Group since its inception in 1995 and under his leadership the company acquired over 30 software firms since 2001.

He was appointed to his latest position in January last year.

Mills, who joined IBM in 1973, had rejected an offer from Hewlett-Packard Co in 2010 to become the software company’s chief executive, Reuters had reported, citing two sources.