On January 16, 2013, Cain won the 3000 meter race at the University of Washington Indoor Preview meet, with what should be a record-setting time of 9:02.10 (high school), superior to the 9:08.6 Outdoor record of Lynn Bjorklund set in 1975. Historically no records set indoors at the University of Washington track have been accepted because at 307 meters, the track is oversized, although it is significantly smaller than a normal 400-meter outdoor track. At the world level since 2000, indoor marks superior to the outdoor record are now accepted. But the record for 3000 meters at the High School level is administered to by Track and Field News which sets its own standards. For the accomplishment, USATF named Cain the Athlete of the Week. On January 26, in New York City, she ran the indoor one mile in 4:32.78. The mark beat Debbie Heald's indoor national high school record from 1972 by almost 6 seconds. Even though she had not started her finishing kick yet, her 1500-meter split time of 4:16.11 surpassed Lynn Jennings' record for that distance from 1978. Her time was also more than two and a half seconds superior to Polly Plumer's outdoor national high school outdoor record from 1982. On February 2, 2013 at the Boston Indoor Games, Cain set the high school record in the indoor 2 mile with a time of 9:38.68. Her time was almost 10 seconds superior to Kim Mortensen's outdoor record for the slightly shorter 3200 meters. Shortly after, at the Millrose Games, Cain ran the mile in 4:28.25. Her time improved upon her own records in the mile and her split time at 1500 also improved upon her records. Her mile time was also more than two seconds faster than Gelete Burka's world youth best. Since 2000, the IAAF recognizes indoor records that are superior to outdoor records, however the IAAF does not track the mile distance at the youth level. On May 17, 2013 at the Oxy Distance Classic, Cain ran the 1500 metres in 4:04.62, shattering her own high school record from the previous year and breaking Suzy Favor's American junior record (who was almost two years older when she set the record in 1987). In her first IAAF Diamond League event in June 2013, Cain set an American junior and high school record in the 800 metres with a time of 1:59.51, surpassing Kim Gallagher's records set in 1982. With her time, she became the first American youth, junior or high school female to break the 2 minute barrier. Cain ran 5,000 meters 15:45.46 in Portland, Oregon at Portland Track Festival on June 8, 2013 to set a United States high school national records in track and field. At the 2013 USA Outdoor Track and Field Championships, Cain entered the 1500 and 5000 metres in order to qualify for the 2013 World Championships in Athletics in Moscow, but later dropped the 5000 from her schedule. In a slow and tactical 1500, Cain led most of the final lap until the last 10 meters when she was passed by training partner Treniere Moser and finished in second place with a time of 4:28.76, thus qualifying for her first senior team as a 17-year-old and becoming the youngest American to ever make a World Championships team. At the 2013 World Championships, Cain qualified for the 1500 metres final by placing 13th in the heats with a time of 4:08.21, and 5th in the semifinals with a time of 4:05.21. In qualifying for the final, she became the youngest athlete ever to run in a 1500 metres final in World Championships history. On November 20, 2013 it was announced that Cain was forgoing a college career in order to run professionally with the Nike Oregon Project. On December 3, 2013 she was named "Youth Athlete of the Year" by USATF. She was also Track and Field News "High School Athlete of the Year."