Thengel was the only son of Fengel, and became king after the death of his father in Third Age 2953.
Fengel was a greedy man, and Thengel left Rohan in his youth and went to live with his mother's family in Gondor. Eventually he met and married Morwen Steelsheen of Lossarnach, and he lived with her there for a while.
After Fengel died Thengel was called upon to return to Rohan, which he reluctantly did. Morwen bore him five children in total (four daughters and a son), including his only son (his second child) Théoden and a daughter Théodwyn, who would be the mother of Éomer and Éowyn.