Like his father Jules, Swaelens taught golf at Royal Latem Golf Club and in 1972, 1973 and 1974 played in thirty tournaments on the recently formed European Tour. He also played in The Open Championship several times. His best result was in 1974 at Royal Lytham & St Annes where he finished tied for seventh.
In 1973, he finished tied for second, with Dale Hayes, in the Viyella PGA Championship held at Wentworth, three shots behind Peter Oosterhuis. In 1974 he finished third in the Dutch Open at Koninklijke Haagsche Golf & Country Club.
Swaelens died in 1975 at the age of 39 from cancer. Until Nicolas Colsaerts played in 2013, he was the only Belgian golfer to be invited to the Masters Tournament, although the cancer prevented him from taking part.
The Donald Swaelens Memorial (later called the Donald Swaelens Challenge) is played each year celebrating his memory. The tournament was originally an 8-man professional tournament sponsored by Laurent-Perrier but is now an amateur tournament.
Winners of the trophy include:
- 1976 Seve Ballesteros
- 1977 Nick Faldo
- 1978 Nick Faldo
- 1993 Lara Tadiotto
- 1995 Arnaud Langenaeken
- 1998 Nicolas Colsaerts (when 15 years old)
- 1958 Gleneagles Hotel Foursomes Tournament (with I McDonald)
- 1967 German Open
- Omnium of Belgium: 1967, 1969, 1970, 1971, 1972, 1973
Results in major championships
|The Open Championship||T11|
|The Open Championship||CUT||23||44||CUT||CUT||T30|
|The Open Championship||CUT||T49||CUT||CUT||T7|
Note: Swaelens only played in The Open Championship.
CUT = missed the half-way cut (3rd round cut in 1968 and 1970)
"T" indicates a tie for a place
- World Cup (representing Belgium): 1958, 1959, 1961, 1962, 1963, 1964, 1965, 1966, 1967, 1968, 1969, 1971, 1972, 1973, 1974
- Joy Cup (representing the Rest of Europe): 1958
- Double Diamond International (representing Continental Europe): 1972, 1973, 1974 (captain)
- Marlboro Nations' Cup (representing Belgium): 1972, 1973
- Sotogrande Match (representing the Continent of Europe): 1974
- A century of golf in Sint-Martens-Latem
- Feron, Hugues (1 April 2009). "Le Royal Latem a cent ans d'histoires". La Libre Belgique (in French).
- Borremans, Vincent (11 May 1998). "On s'est remémoré l'ancien champion Nicolas Colsaerts a remporté le "Donald Swaelens"". Le Soir (in French).
- Borremans, Vincent (9 May 1997). "Les amateurs belges disputent ce week-end le challenge Donald Swaelens, l'hommage à un champion modeste". Le Soir (in French).
- "Donald Swaelens". The Times. 29 April 1975. p. 18.