He had not previously been interested in math, although his father was the head of the University of Western Ontario's mathematics department and his mother is associate dean of medicine there.
While he was there, he, his brother Jonathan Borwein and David H. Bailey of NASA wrote the 1989 paper[1] that outlined and popularized a proof for computing one billion digits of π.
In 1993, he moved to Simon Fraser University, joining his brother Jonathan in establishing the Centre for Experimental and Constructive Mathematics (CECM) where he developed the Inverse Symbolic Calculator.
Borwein has developed an algorithm that applies Chebyshev polynomials to the Dirichlet eta function to produce a very rapidly convergent series suitable for high precision numerical calculations, which he published on the occasion of the awarding of an honorary doctorate to his brother, Jonathan.
[2] Peter Borwein also collaborated with NASA's David Bailey and the Université du Québec's Simon Plouffe to calculate the individual hexadecimal digits of π.