Martha Nelson, a former editor in chief of Time Inc., will join Yahoo as its global editor in chief, the company said Thursday, overseeing its portfolio of digital magazines. Ms. Nelson, the founding editor of InStyle and a former editor of People magazine, had taken the top editorial job at Time Inc. in 2013. But she left later that year in a shake-up that eliminated her position and had editors of the company’s 21 magazines reporting to the business side. She will be based in New York when she starts her new job next week, Yahoo said, and will focus on developing Yahoo’s 13 domestic digital magazines and 30 international magazines.