The deal marks a break for McCartney from EMI, which was acquired for $2.2 billion by a group of investors led by Sony/ATV last month, making it the world's biggest music publisher.
The move ensures different managers for McCartney's Beatles and non-Beatles hits in markets not already served by MPL.
It also represents the growing popularity of Kobalt among artists.
According to Billboard magazine, Kobalt increased its share of publishing revenue from the top 100 U.S. radio hits to 15.5 per cent in the third quarter of 2011, up from less than 5 per cent three years earlier. That ranked it second behind only EMI, which had 17.9 per cent at the time.
Kobalt says it pays songwriters about 25 per cent more than other publishers because of its streamlined structure and better tracking of song plays around the world.
"We have grown by delivering service, quality, and results to our clients. We have not bought our growth. We have earned it," said Kobalt CEO and founder Willard Ahdritz in an interview.