For the first part of his career, after he became Vienna Chess Champion in 1861-62, Steinitz was no stronger than at least half a dozen of his contemporaries. He was 6th in the London tournament of 1862. He stayed in London and, although clearly one of the best players in the World, there were at least half a dozen others who were on the par with him. By the end of the 1860s at least two of his main rivals (Burn and Lowenthal) considered him to be the strongest player in the World. He had achieved this status as a result of playing sharp, tactical openings but by the Vienna tournament of 1873 he had changed his style completely.
At the Vienna tournament, Steinitz's style became positional, and the resulting transformation was that other players were simply unable to cope. His score in that event was 18 wins, 5 draws and 2 losses. During the next nine years, Stenitz played little chess, but in 1883 he was second to Zukertort in a tournament in London. Although Steinitz had defeated Zukertort in 1872, some regarded Zukertort as the strongest player in the world. So it was decided that a match should be played and the first to win 10 games should be declared World Champion. This match took place in 1886 in the cities of New York, St. Louis and New Orleans. Steinitz was the winner with 10 wins, 5 draws and 5 losses, so he became the first World Champion at the age of 49 years. He kept the title until 1894.
Click here for a good website where you can download Stenitz's games in .pgn format.
To see one of Steinitz's greatest games fully annotated click here