Decleir made his debut as film actor in 1971 production Mira of Fons Rademakers, in wich he played the farmer’s son as opponent of Willeke van Ammelrooy. In the world of theatre he became famous by acting with the International Scene for the theatrical piece Mistero Buffo if Dario Fo. He travelled around the world with his monologues Obscene Fables of Dario Fo. The television audience got to know Decleir when he played the role of Sil the beachcomber in 1976 in the television series which wears the same name.
Children know Decleir as Saint Nicholas from Dag Sinterklaas (1993), that was broadcasted during each Saint Nicholas period on Ketnet. In 2005 was Decleir again a part of the Saint Nicholas team by taking back his role in the family film Het Paard van Sinterklaas. He also played king Joseph in the series Kulderzipken.
Other outstanding roles did Decleir play in for instance the film Daens (1993), in which he portrayed the priest Adolf Daens and which was nominated for an Oscar, Camping Cosmos (1996) of Jan Bucquoy in which he played the lover of Harry, Karakter, the film of Mike van Diem that won an Oscar (1997), Retour Den Haag (televisie, 1999), in wich he played the role of president Ruud Lubbers, Kruimeltje (1999) as the strict father Keyzer of an orphanage, Negen dagen van de gier (television, 2001) and Stille Waters (2001). In 2003 he played the role of Angelo Ledda, a hit man with Alzheimer’s disease in the film De zaak Alzheimer. In 2006 he played the role of 'De Generaal' in De Kavijaks, a five-part series broadcasted by Netherlands’ broadcasting KR] and later on also on the Flemish television channel VTM. His children also played a leading role in this series: Jenne Decleir (1977) and Sophie Decleir (1970).
Jan Decleir received multiple awards for his work. The film Antonia and “Karakter” both received an Oscar. Daens was also nominated for the Oscar for Best Foreign Language Film. During the film festival in Montréal he received the award for Best Actor for his role in the by Pieter Kuijpers directed film Off Screen. At the Netherlands Film Festival he received a Gouden Kalf for his significance for Dutch-language film. In 2008, he received the Best Actor award at the Tiburon International Film Festival for his role in Man zkt Vrouw.
Decleir was several years in charge of Studio Herman Teirlinck. He renewed the famous drama school and brought in many new teachers: actors from the Blue Monday Company, STAN and others. He could not prevent that new austerity measures would hinder the school operation.