He was born in Vilnius, Polish-Lithuanian Commonwealth (now in Lithuania), in 1762. At an early age, he was violinist at the Polish Royal Chapel, after which he travelled to Vienna about 1785 to see and listen to Haydn and Mozart. From Austria, he accompanied a Polish princess to Italy, to refine his artistic abilities with the Italian masters of that day, where he lived for three years. In Paris, he appeared at the Concerts Spirituels in December 1787, and was described in the Mercure de France as a pupil of the Italian violinist Giovanni Giornovich and was immediately recognised by the Parisians as an artist of high rank. There he was employed by the Duc d'Orléans and for a short time he enjoyed the pension of a musician on the establishment of Mlle. d'Orléans; on the outbreak of the revolution he left France for London in 1790 and did not reappear until his 1792 London debut at Salomon's Concerts among others. 
During the subsequent years he expanded his violin performances and musical pursuits to Manchester, Liverpool, and Ireland. In Liverpool, he met Miss Breeze, whom he married in 1800.
Janiewicz played at Corri's house in London in January 1792, and at Growetz's concert on 9 February, giving a benefit concert in the same month. He performed his violin concerto at the Saloman concerts of 17 February and 3 May (for Haydn's benefit). During several seasons Janiewicz played in London, visited the provinces and Ireland as a violinist, and conducted the subscription concerts in Manchester and Liverpool. He was one of the original members of the London Philharmonic Society, and in the first season (1813) was one of the leaders of the orchestra. 
For a time he kept a music-warehouse at 25 Lord Street, Liverpool, and married Miss Breeze of that town in 1800. In 1815 he went to Edinburgh. 
He is buried alongside his wife, Eliza in Warriston Cemetery, Edinburgh. The grave lies on the west side of the western path, on its lower half.
- 5 Violin concertos (1788-1803)
- Piano concerto
- 'Six Divertimentos for Two Violins,' London, 1800?
- 'Sonata for the Pianoforte, with Accompaniment for the Violin,' in which is introduced Handel's 'Lord, remember David,' London, 1800?
- 'Go, youth belov'd,' song, Liverpool, 1810?
- 'Polish Rondo for Pianoforte,' Liverpool, 1810?
and many adaptations.
- Pontigny 1906.
- Edinburgh and Leith Post Office Directory 1857-8
Pontigny, Victor de (1906). "Janiewitz, Felix". In J. A. Fuller Maitland (ed.). Grove's dictionary of music and musicians. 2. New York. p. 525.