Born on 14 April 1911 at Bobrka. Having passed the final certificate exams and completed a general military course, he joined the Air Force Cadet Officers’ School in Deblin. He was commissioned in 1934 as an observer. He was posted to the 1st Air Regiment in Warsaw. He completed flying training at the Air Force Officers Training Centre in Deblin. He was a pilot with the 112th Fighter Flight. In 1938 he was posted to the Advanced Flying Training School in Grudziadz as an instructor (in March 1939 the school moved to Ulez near Deblin). In September 1939 he fought in the Deblin Group, defending the airfield area. He got to France via Rumania. He fought in a section defending Clermont-Ferrand, under Maj. Walerian Jasionowski. After the armistice W. Zak was evacuated to Britain. After conversion training on new equipment he was assigned to the newly formed No. 303 (Polish) Squadron 'T. Kosciuszko'. On 15 September 1940 he was credited with a Do 17 destroyed, and on 26 September with a He 111. On 27 September 1940 he was shot down in air combat, burnt and wounded he was taken to a hospital. On 1 February 1941 he was decorated with Cross of Valour and bar. On 9 July 1941 he was appointed a flight commander in No. 303 Squadron. He remained at that post until 4 December 1941. On that day he was posted as an instructor to No. 58 OTU at Grangemouth. On 7 April 1942 he returned to No. 303 Squadron as a flight commander. On 10 May 1942 he was appointed commander of the squadron. He commanded it until 25 May 1942. From 26 May 1942 he commanded No. 308 (Polish) Squadron 'City of Cracow'. On 15 November 1942 he was awarded the Silver Cross of Virtuti Militari Order. On 11 February 1943 he became the Polish Liaison Officer to HQ No. 12 Group. From 29 June 1943 he commanded the newly formed 3rd Polish Fighter Wing. As part of the establishment of the 2nd TAF No. 133 (Polish) Wing was formed and W/Cdr Walerian Zak became its first Wing Leader (from 12 October 1943 until 15 February 1944). On 16 November 1943 he was decorated with Distinguished Flying Cross. On 15 February 1944 he was posted to HQ No. 11 Group. On that day he also received a second bar to his Cross of Valour. On 24 April 1944 he was posted to the Air Defence of Great Britain. From 8 January 1945 he served at No. 61 OTU and was then posted to the Polish Air Force Staff College. Having graduated from it he was posted as the Polish Liaison Officer to HQ 2nd Tactical Air Force. From 21 June 1945 he led No. 133 (Polish) Wing, and from 8 August 1945 the 2nd Polish Fighter Wing at Coltishall. Then he was an administration officer at No. 5 RU. He retired from the Polish Air Force in 1947. He settled in Britain. He died on 13 March 1969 in Surbiton.