[1] Fjellstedt was born to carpenter Lars Larsson and Catharina Carlsdotter in Värmland, Sweden in 1802, the first child in a poor family of craftsmen.

He then studied in Karlstad for three and a half years, renting living space from a tavernkeeper and having to sleep on the lid of a wooden chest with his coat for a blanket.

"[5][1] He later stayed in Småland for over a year and a half working for Lieutenant Colonel C. Kuylenstierna and Major C. J. von Mentzer as a private tutor, where he experienced a strong spiritual revival.

On his way there, he made a visit to Sweden, where he met Peter Wieselgren and led to the founding of Svenska missionssällskapet i Göteborg ('the Swedish Mission Society in Gothenburg').

After working for a time as a traveling preacher in Switzerland, Germany and France, Fjellstedt returned to Sweden in 1843, where he was involved in teaching, writing and training.

One priest, P. Wagenius, after hearing Fjellstedt's preaching likely in 1857, described the scene:[1] "When I go back even further in my memory and search for such heavenly impressions, which are sweet to the spirit, I remember a beautiful summer evening in the Östersund church, when the western sun shone through the windows and shone around the man of God, who then stood on the pulpit and testified about the divine, glorious world mission.

With the most joyful assurance and rapture he recited and applied many, many of the prophetic promises about the time when the earth will be filled with the knowledge of the Lord, just as the waters cover the bottom of the sea.

His appearance, his cheerful assurance of faith, his clear, glorious presentation, made me think I saw and heard one of the Old Testament prophets standing there before the Lord, proclaiming the revelations he had received from Him.

[9] A number of free-church colporteurs, itinerant sellers of Christian literature, were involved early on; however, many left to form the Baptist Union of Sweden in 1857.

Towards the end of his life he was engaged in writing; his Biblia, det är all den Heliga skrift, med förklaringar was one of Baptist publisher Per Palmqvist's most significant publications.

Around 1850 he wrote to the missionary inspector Blumhardt in Basel, "The state of Christianity in Sweden is very deplorable, and the cause is an ignorance which surpasses all imagination, and that among all classes of people."

Fjellstedt placed an emphasis on Christian eschatology, holding a historic premillennial, non-dispensationalist view on the Second Coming of Jesus, which he preached on.

His work Bibliska framtidsvinkar also addresses these topics, discussing the signs of the times and the date of the return of Christ as well as the Antichrist and the apostasy of the Jews.