The mycobacterial outer membrane is composed of unusual lipids and glycolipids. Some of these lipids are exported to the cell envelope by resistance-nodulation-division (RND) transporters called mycobacterial membrane protein large (MmpL). While the oligomeric state of most RND transporters is well established, MmpL assembly remains unclear. Here, we investigated MmpL10, the trehalose polyphleate transporter. Biochemical data suggest that MmpL10 forms a homotrimer and that its oligomerization is driven by a coiled-coil domain. Structural modeling and electron microscopy data reveal the presence of a tubular extension that spans the mycobacterial cell wall and reaches the mycomembrane. As most MmpL proteins possess this extension, oligomerization may be a common feature of this family of transporters, possibly involved in the transport of the MmpL cargo.