Jerry Yang rode Yahoo Inc.'s swift rise and subsequent decline over 17 years. Now the co-founder of the onetime dominant Internet company has decided to stop hanging on.

Mr. Yang, 43 years old, said Tuesday he was resigning from Yahoo's board, severing ties to the company he co-founded with David Filo in 1995 while both were Stanford University graduate students. Mr. Yang also said he would leave the boards of Yahoo Japan Corp. and Alibaba Group Holding Ltd., in which Yahoo owns significant stakes.

Mr....