diff --git a/src/init.cpp b/src/init.cpp
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -1384,11 +1384,20 @@
 
         // scan for better chains in the block chain database, that are not yet
         // connected in the active best chain
-        BlockValidationState state;
-        if (!ActivateBestChain(config, state)) {
-            LogPrintf("Failed to connect best block (%s)\n", state.ToString());
-            StartShutdown();
-            return;
+
+        // We can't hold cs_main during ActivateBestChain even though we're
+        // accessing the g_chainman unique_ptrs since ABC requires us not to be
+        // holding cs_main, so retrieve the relevant pointers before the ABC
+        // call.
+        for (CChainState *chainstate :
+             WITH_LOCK(::cs_main, return g_chainman.GetAll())) {
+            BlockValidationState state;
+            if (!chainstate->ActivateBestChain(config, state, nullptr)) {
+                LogPrintf("Failed to connect best block (%s)\n",
+                          state.ToString());
+                StartShutdown();
+                return;
+            }
         }
 
         if (gArgs.GetBoolArg("-stopafterblockimport",
diff --git a/src/qt/test/apptests.cpp b/src/qt/test/apptests.cpp
--- a/src/qt/test/apptests.cpp
+++ b/src/qt/test/apptests.cpp
@@ -104,7 +104,7 @@
     // Reset global state to avoid interfering with later tests.
     AbortShutdown();
     UnloadBlockIndex();
-    g_chainman.Reset();
+    WITH_LOCK(::cs_main, g_chainman.Reset());
 }
 
 //! Entry point for BitcoinGUI tests.
diff --git a/src/validation.h b/src/validation.h
--- a/src/validation.h
+++ b/src/validation.h
@@ -1134,14 +1134,41 @@
     //! free the chainstate contents immediately after it finishes validation
     //! to cautiously avoid a case where some other part of the system is still
     //! using this pointer (e.g. net_processing).
+    //!
+    //! Once this pointer is set to a corresponding chainstate, it will not
+    //! be reset until init.cpp:Shutdown(). This means it is safe to acquire
+    //! the contents of this pointer with ::cs_main held, release the lock,
+    //! and then use the reference without concern of it being deconstructed.
+    //!
+    //! This is especially important when, e.g., calling ActivateBestChain()
+    //! on all chainstates because we are not able to hold ::cs_main going into
+    //! that call.
     std::unique_ptr<CChainState> m_ibd_chainstate;
 
     //! A chainstate initialized on the basis of a UTXO snapshot. If this is
     //! non-null, it is always our active chainstate.
+    //!
+    //! Once this pointer is set to a corresponding chainstate, it will not
+    //! be reset until init.cpp:Shutdown(). This means it is safe to acquire
+    //! the contents of this pointer with ::cs_main held, release the lock,
+    //! and then use the reference without concern of it being deconstructed.
+    //!
+    //! This is especially important when, e.g., calling ActivateBestChain()
+    //! on all chainstates because we are not able to hold ::cs_main going into
+    //! that call.
     std::unique_ptr<CChainState> m_snapshot_chainstate;
 
     //! Points to either the ibd or snapshot chainstate; indicates our
     //! most-work chain.
+    //!
+    //! Once this pointer is set to a corresponding chainstate, it will not
+    //! be reset until init.cpp:Shutdown(). This means it is safe to acquire
+    //! the contents of this pointer with ::cs_main held, release the lock,
+    //! and then use the reference without concern of it being deconstructed.
+    //!
+    //! This is especially important when, e.g., calling ActivateBestChain()
+    //! on all chainstates because we are not able to hold ::cs_main going into
+    //! that call.
     CChainState *m_active_chainstate{nullptr};
 
     //! If true, the assumed-valid chainstate has been fully validated
@@ -1205,7 +1232,7 @@
     void Reset();
 };
 
-extern ChainstateManager g_chainman;
+extern ChainstateManager g_chainman GUARDED_BY(::cs_main);
 
 /** @returns the most-work valid chainstate. */
 CChainState &ChainstateActive();
diff --git a/src/validation.cpp b/src/validation.cpp
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -62,11 +62,13 @@
 ChainstateManager g_chainman;
 
 CChainState &ChainstateActive() {
+    LOCK(::cs_main);
     assert(g_chainman.m_active_chainstate);
     return *g_chainman.m_active_chainstate;
 }
 
 CChain &ChainActive() {
+    LOCK(::cs_main);
     return ::ChainstateActive().m_chain;
 }
 
@@ -790,6 +792,7 @@
 static CBlockIndex const *pindexBestForkBase = nullptr;
 
 BlockMap &BlockIndex() {
+    LOCK(::cs_main);
     return g_chainman.m_blockman.m_block_index;
 }
 
@@ -5187,7 +5190,7 @@
                 // continue
                 if (hash == chainparams.GetConsensus().hashGenesisBlock) {
                     BlockValidationState state;
-                    if (!ActivateBestChain(config, state)) {
+                    if (!ActivateBestChain(config, state, nullptr)) {
                         break;
                     }
                 }