Due to the passing of his uncle, the 12th lord of the domain named Narikatsu TOKUGAWA, Iemochi was adopted to take over the family estate when he was four years old and became the 13th lord of the domain in 1849.
嘉永2年（1849年）に叔父で第12代藩主である徳川斉彊が死去したため、その養子縁組として家督を4歳で継ぎ、第13代藩主となった。