Tick-borne diseases are considered a major emerging health problem around the world, affecting more than one billion people and accounting for over 700,000 deaths annually. Midichloria mitochondrii (Family Midichloriaceae; Order Ricketsiales) are obligate, intracellular endosymbionts of disease-causing Ixodes ticks, including the castor bean tick Ixodes ricinus and the Australian paralysis tick Ixodes holocyclus. M. mitochondrii are observed in abundance throughout the tick body, with strong localisation in the ovaries, found in both the cytoplasm and within the inter-membrane space of the mitochondria. Although abundantly present, the biological role of M. mitochondrii is not well understood. To address this gap in our knowledge, metatranscriptomic and differential expression analyses of M. mitochondrii, as well as the identification of lateral gene transfer (LGT) events between the endosymbiont and its host, were undertaken in the present study. Classifying the reads from the metatranscriptomic analyses by functional groups revealed that a high proportion of reads were involved in translation, ribosomal structure and biogenesis, followed by posttranslational modification, protein turnover and chaperone activity. Differential expression analysis additionally showed no statistical significance of M. mitochondrii expression between tick life stages, similar to results from studies of other invertebrate bacterial endosymbionts. Lastly, several candidate LGT events were observed between M. mitochondrii to I. ricinus, two of which had lengths >100 nt. Expression of these two LGT suggested that these LGT events are used by the tick host, however their function remains unclear. This study enhances our understanding of the activity of M. mitochondrii and shows very similar biological roles between varying life stages due to a general lack of gene regulation. Better understanding the endosymbionts of disease-causing hosts, such as Ixodes ticks, is important for developing control mechanisms and reducing the spread of tick-borne diseases.