He was a prolific portrait painter, creating works both fine and mediocre, though most were competently done. Some were never completed, and a few of those are the subject of this post.
I find unfinished works interesting because they shed light on artists' techniques and general approach to the job. In Lawrences's case, he invariably completed the face first, along with enough background to put the colors in intended context. The remainder would be very roughly indicated.
One strongly recommended approach to painting is to work the entire canvas throughout the process. This indeed makes a lot of sense when painting landscapes or still-lifes. But a portrait painter needs to be sure the subject's face is captured to his (and probably his sitter's) satisfaction. So why waste time and paint working the whole canvas if it turns out that the face isn't done right? That seems to have been Lawrence's philosophy if the paintings shown below are any indication.
This can be seen in London's National Portrait Gallery.
Also in the National Portrait Gallery. Here Lawrence sketched in more non-facial detail than usual, perhaps due to the size and shape of the canvas. He probably wanted to make sure he got the overall composition right, something not needed on more tightly focused subjects.
Painted not long before Lawrences' death, so perhaps he didn't have the time or energy to complete it.
The same hold true for this portrait of the Duke of Wellington as Prime Minister. It is a recent acquisition by the National Portrait Gallery. The caption on the painting's plaque notes that Lady Jersey, who commissioned the portrait, refused to have a studio assistant complete it following Lawrences's death.