He was born to Gongsun Du, Governor of Liaodong. In 204 Kang took over the position with the territories of Liaodong, Xuantu and Lelang. He was nominally subject to Cao Cao while keeping his domain semiindependent. In 207 he pleased Cao Cao in killing Yuan Shang (袁尚), the last attested member of the clans of Yuan Shao and escaped the conquest of Cao Cao to Liaodong.
He smashed Yiyimo (伊夷模), King of Goguryeo, at her capital and forced him to move the capital. He separated the southern half from the Lelang commandery and established the Daifang commandery in 204 to make administration more efficient. With military expeditions he brought southern natives into submission.
When he died in 221, his younger brother Gongsun Gong (公孫恭) succeeded him because Kang's children were young. However, Kang's son Gongsun Yuan took back the position in 228.