McKerr, 18, has represented South Africa at Under-19 level but holds a British passport.
Pillans, 24, joins from the Dolphins and will play for the county under an ancestral visa.
"Following the retirements of Chris Tremlett, Tim Linley and David Balcombe we needed to add to our resources," said director of cricket Alec Stewart.
"Conor is a part of this process and Mat will add quality and strength to our seam bowling unit."
Pretoria-born Pillans has taken 114 wickets at an average of 23.72 in 31 first-class appearances.
The right-armer, who has recently been playing for KwaZulu-Natal Inland, will link up with Surrey in the next fortnight.
Surrey begin their County Championship campaign against Nottinghamshire on Sunday, 10 April.
Surrey have signed South African fast bowlers Conor McKerr and Mathew Pillans on three-year deals.