A modular exhaust after-treatment system including a plurality of exhaust after-treatment modules. Each module includes a housing defining a non-linear flow path arranged between an inlet passage and an outlet passage. The non-linear flow path includes a first portion adjacent the inlet passage that includes a first exhaust treatment component, a second portion downstream from the first portion, and a third portion downstream from second portion adjacent the outlet passage, wherein the third portion includes a second exhaust treatment component. The plurality of exhaust after-treatment modules are secured to each other in either horizontally or vertically.