At the age of 16, he quit his study at the Hogere burgerschool and started to work for Unilever. In 1974 he started to work for Sara Lee, where he ultimately became the president of the Board of Directors.
On 31 December 1993 he withdrew from Sara Lee. In the beginning of 1994 he was asked by former Philips top executive Jan Timmer to participate in the Board of Directors of Philips, to give a new imulse to the brand Philips. Boonstra was responsible for the 'Let's make things better'- campaign.
Two years after his commencement of employment with Philips, Boonstra became the successor of Timmer as the president of the Board of Directors. He held this leading position until 2001. While Boonstra was in charge, the market-value of Philips increased fivefold. During his leadership the head office of Philips moved from Eindhoven to Amsterdam. In 2000 Boonstra was chosen 'top executive of the year'. Boonstra divorced two times, and has two children (Cor Boonstra and Roelof Boonstra) of his first wife.