Proteoform-resolved information, obtained by top-down (TD) ″intact protein″ proteomics, is expected to contribute substantially to the understanding of molecular pathogenic mechanisms and in turn, identify novel therapeutic and diagnostic targets. However, the robustness of mass spectrometry analysis of intact proteins in complex mixtures is hindered by high dynamic range in protein concentration and mass, protein instability, and the chemical complexity of biological samples. Here, we describe an evolutionary step for intact protein investigations through the online implementation of tandem microflow size exclusion chromatography with nanoflow reversed-phase liquid chromatography and mass spectrometry (μSEC2-nRPLC-MS). Online serial high- and low-pass SEC filtration overcomes the aforementioned hurdles to intact proteomic analysis through automated sample desalting/cleanup and enrichment of target mass ranges prior to nRPLC-MS analysis. The coupling of μSEC to nRPLC is achieved through a novel injection volume control (IVC) strategy of inserting protein trap columns pre- and post-μSEC columns to enable injection of dilute samples in high volumes without loss of sensitivity or resolution. Critical characteristics of the approach are tested via rigorous investigations on samples of varied complexity and chemical background. Application to cerebrospinal fluid (CSF) samples pre-fractionated by OFFGEL isoelectric focusing demonstrates that the platform drastically increases the number of intact mass tags detected within the target mass range (< 30 kDa) in comparison to one-dimensional nRPLC-MS starting from approximately 100x less CSF than previous studies. Furthermore, the modular design of the μSEC2-nRPLC-MS platform is robust and promises significant flexibility for large-scale TDMS analysis of diverse samples either directly or in concert with other multidimensional fractionation steps.