A graduate of Cornell U, he joined the US Weather Bureau at Pittsburgh in 1898. In 1900 he took part in the first use of the telegraph for reporting local weather. Thiessen was stationed on Cobb Island in the Potomac River. On receiving a telegraph command to report the local weather, he duly replied to base (just 1 km distant!) that it had just started snowing. He resigned from the Bureau in 1920 to join the US Army, retiring as a Major. He rejoined the Bureau for a year in 1941.