After leaving the Grammar School, Tecwyn began an Engineering Apprenticeship with Saunders-Roe, at Fryars Point, Beaumaris, Anglesey (some eight miles from Llanddaniel) where Catalina flying boats were converted and maintained. The Menai Straits provided an excellent location for operations and the flying boats were hauled out on the slipway, which remains on the foreshore today, along with most of the wartime hangars.

After Tecwyn’s release from the RAF in 1944, he resumed work with Saunders Roe at their Southampton works, and then transferred to the Isle of Wight in 1946, where he met his wife Doris. He studied at Southampton University and gained his Higher National Certificate in Aeronautical Engineering in 1948, and was also awarded the Institute of Mechanical Engineers Special Award for his achievement.

In December 1952 Tecwyn and his wife, Doris, sailed for Toronto, where he was involved in designing the Canadian fast fighter jet, the Avro Arrow. In 1959, the sudden cancellation of the Arrow project by the Canadian government led to massive redundancies. Thirty-two of the finest Avro engineers, amongst them Tecwyn Roberts, were head-hunted by the newly formed American Space Task Group, based at the Langley Research Center in Hampton, Virginia. Their task was to kick-start America’s manned space-flight programme.

Tecwyn became NASA's first Flight Dynamics Officer at the Mercury Control Center at Cape Kennedy. He played a key role in formulating the requirements for the tracking and communications network, and in controlling the trajectory of the spacecraft and planning adjustments to it. He was also instrumental in the design of the Centre and also the subsequent Mission Center established in Houston, Texas, later re-named The Johnson Space Centre. For his accomplishments in that area, he received the NASA Outstanding Achievement Award. Tecwyn was therefore inextricably linked to the success of Project Mercury in the early 1960s to orbit a manned spacecraft around Earth; to investigate man's ability to function in space; and to recover both man and spacecraft safely.

In 1962, Tecwyn Roberts was appointed head of Manned Flight Division at the Goddard Space Flight Center, Maryland, where his son, Michael, was born. He later became chief of the Manned Flight Engineering Division, putting him in charge of NASA's Manned Space Flight Network, a set of tracking stations built to support the American space efforts of Mercury, Gemini, Apollo and Skylab.  For this achievement, he was honoured in 1964 with the NASA Exceptional Service Award for his contribution to the manned space flight program in the area of flight operations.

In 1965, during the Apollo Programme, Tecwyn became the chief of the Manned Flight Support Division at the Goddard Space Flight Center, and then in 1967, Chief of the Network Engineering Division, which he headed during the first landing on the moon. In 1969, he received the NASA Exceptional Service Medal for his work in support of the Apollo 8 flight. Tecwyn’s last direct involvement with NASA’s manned space flights was as Director of Networks at Goddard and his  role in  ensuring that contact was maintained between the orbiting  U.S. and Soviet spacecraft in the Apollo – Soyuz Project. In 1975 he was elected Fellow of the American Astronautical Society. 

In 1979, Tecwyn Roberts retired from NASA  and in  1984, he was honoured with  the Robert H. Goddard Award of Merit, the highest level of recognition the Goddard Space Center can bestow on its employees. Four years later, Tecwyn Roberts died on 27 December 1988, aged 63 years. He was buried at St. Stephens Episcopal Cemetery, Crownsville, Maryland.