Control of bacterial adhesion and motility is central to both microbial engineering and bio interface design. In Escherichia coli, Type 1 fimbriae (T1F) and flagella represent two key surface structures that regulate transitions between motile and sessile states. T1F are phase-variable appendages whose expression is controlled by the reversible inversion of the fimS promoter region, allowing populations to dynamically alternate between adhesive and planktonic modes. In this work, we investigated how genetic manipulation of the T1F operon (fimA–H) alters the balance between attachment and motility in two E. coli strains: K12 MG1655 and Nissle 1917 (ECN). Using targeted mutagenesis, three fimbrial variants were constructed in each background: a complete T1F deletion mutant (ΔfimA–H), a phase-locked-ON strain constitutively expressing T1F, and a phase-locked-OFF strain with T1F irreversibly repressed. These engineered strains were characterised through motility assays, transcriptional profiling, and surface adhesion tests on previously uncharacterised biological substrates. The resulting phenotypes revealed that modifications to fimbrial regulation influence not only attachment strength but also flagellar activity and surface interaction dynamics. Comparative analysis of MG1655 and ECN suggests that strain-specific regulatory networks modulate this relationship, shaping how they engage with their environment. Together, these results demonstrate that targeted genetic control of fimbrial expression can tune bacterial surface behaviours, providing a mechanistic insight and framework for engineering microbial systems with custom adhesion or motility properties. This work underscores the potential of integrating genetic and surface-level engineering approaches to design bacteria optimised for biotechnological, environmental, or biomedical applications.