Silas Kiplagat defeated Mwangangi at the Prisons Cross Country in 2011, but he rebounded by lifting the inaugural men's senior title at the 2011 African Cross Country Championships. That same month he ran at the Lisbon Half Marathon and came runner-up behind Zersenay Tadese, who ran the then-second-fastest time ever. He defended his title at the Rabat Half Marathon a month later. In the second half of the season, he came third at the Dam tot Damloop in a 10-mile best time of 45:13 minutes. He won for a second consecutive time at the Paris 20K then completed the same feat at the Valencia Half Marathon, where he improved his best over the distance to 59:45 minutes. In December he came third at the Montferland Run. He was runner-up to Zersenay Tadese at the Lisbon Half Marathon in March 2012. He was one of the pacemakers at the 2012 Rotterdam Marathon and the top two runners finished under two hours and five minutes. At the 2012 World Half Marathon Championships in Kavarna, Bulgaria, he won the bronze medal in a time of 1:01:01 hours, and helped his teammates to win the team title. His last outing over that distance that year, saw him finish as runner-up in Valencia.