(In 1892 he received a Congressional medal of honour for "conspicuous gallantry at the battle of Wilson's Creek.") In1861-1863he performed various military duties in Missouri.
For his work on etherification Williamson in 1862 received a Royal medal from the Royal Society, of which he became a fellow in 1855, and which he served as foreign secretary from 1873 to 1889.
In 1901 the Copley medal of the Royal Society of London was awarded him as being "the first to apply the second law of thermodynamics to the exhaustive discussion of the relation between chemical, electrical and thermal energy and capacity for external work."
The Copley medal was conferred upon him in 1823, and the Lalande prize in 1817 by the Paris Academy, of which he was a corresponding member.
He was especially interested in questions relating to the polarization of light, and his observations in this field, which gained him the Rumford medal of the Royal Society in 1840, laid the foundations of the polarimetric analysis of sugar.