There are a number of Collins biographies. One of the most recent is called "The King of Inventors" by Catherine Peters (thanks to one of my corresponents for this), but I haven't read it yet. The following is culled from a number of sources:
William Wilkie Collins was born in London on 8 January 1824, the oldest son of William Collins, RA, a fashionable painter. Collins was named after a friend of the family, Sir David Wilkie, RA.
His early career was in commerce, but he then studied law and was actually admitted to the Bar, but does not appear to have practised. He had been writing articles and stories for some time, many of which were published (though apparently anonymously). He also seems to have tried his hand at painting, and in 1848 had a picture exhibited in the Royal Academy.
His first major published work was a biography of his father, and the popularity of this book seems to have encouraged him to write seriously for a living. His first novel, Antonina or The Fall of Rome was published in 1850. This was a historical work, along the lines of Bulwer-Lytton's The Last Days of Pompeii. It appears to have been quite successful, but is today pretty well unreadable. (My 1885 copy still has its pages uncut!).
In 1851, Collins was introduced to Charles Dickens by a mutual friend, Augustus Egg. This was a seminal point in Collins' career. The two formed a long-lasting friendship, and collaborated on Dickens' magazines, Household Words, and All the Year Round, in which many of their works were serialised.
Both shared an interest in the theatre, and indeed many of Collins' works appear to have started out as plays (many have a Dramatis Personae, for example, and have a 'stagey' feel).
Collins appears to have led a rather strange marital life. He lived with a lady called Caroline Graves and her daughter by a previous marriage for most of his life, but never married her. Then abruptly, she married someone else, and Collins took up with Martha Judd, by whom he had three children, but again never formally marrying her. Towards the end of his life he took up again with Caroline Graves. (There is a whole book speculating about this side of Collins' life, called "The Secret Life of Wilkie Collins", see Books About Collins).
As he grew older, Collins suffered terribly from gout, for which he took laudanum. Most of The Moonstone was dictated while he was bed-ridden and in agony.
He died in 1889.