Mixed alkali borosilicate glasses show a complex relationship between composition and structure. These relationships are often difficult to fully investigate using standard laboratory methods. Therefore, we present here a systematic molecular dynamics (MD) study of composition–structure relationships in sodium lithium borosilicate glasses. Results are compared with the published laboratory glass data. These glasses are of importance to the U.K. nuclear waste vitrification program where an understanding of glass structure can improve greatly the understanding of long-term glass properties and waste element solubility. The MD-simulated glasses produced in this work show close agreement with laboratory glasses for bond lengths, atom coordinations and bond angles of all species. Particularly, the change in coordination of boron is accurately reproduced over a wide compositional range without the use of a three-body potential for the O–B–O angle. The change in the polymerization and mixing of the silicate and borate networks with composition in the MD-simulated glasses was shown to agree well with that seen in laboratory glasses.