Lawes was the only son of the lord of the manor of Harpenden in Hertfordshire and inherited his father's estates in 1822. He was educated at Eton and Oxford University, but he left without taking a degree. He developed an interest in science, particularly chemistry, and at the age of 20 he constructed a laboratory for himself at his home.

He turned his attention to the problems of agricultural chemistry when a neighbor pointed out to him that on some local farms bone meal increased turnip production, while on others it seemed to have no effect. This started him on his life's work, the chemistry of fertilizers.

After experimentation, Lawes showed that it was necessary to make the phosphate in the bones more readily soluble in the soil for absorption by plants. This he achieved by adding sulfuric acid to the crushed bones. Lawes took out a patent on these ‘superphosphates’ in 1842, opening his first factory for their production in 1843. By the 1870s he was producing 40,000 tons of superphosphates a year using phosphate rock rather than bones.

Also in 1843, Lawes was joined by Henry Gilbert, beginning a lifelong collaboration, and he started the Rothamsted Experimental Station, the first agricultural research station in the world. Experiments were conducted on different fertilizers; crops which were normally grown in rotation were grown here year after year on the same plot using a variety of manures and fertilizers. Animal feed was also examined and varied to find the most economical and efficient. Well over 100 papers were produced by Lawes and Gilbert on their Rothamsted work.

Lawes established the Lawes Agricultural Trust in 1889 to safeguard the continuation of research following his death. He was created a baronet in 1882.